The purpose of this experiment is to test the effects on tolerance and craving of a constant cocaine dose delivered with two infusion intervals: 12 and 24 hours. It is hypothesized that continuous exposure for 24 hours to cocaine at a dose which produces minimal subjective or physiological effects will induce tolerance and diminish craving for cocaine. In contrast, shorter infusion periods will not affect tolerance or craving. Tolerance and craving will be assessed by comparing the responses to test probe doses of smoked cocaine administered before and following cocaine infusions. Craving will be assessed with specific craving psychometric instruments. To ensure subject compliance and safety, the subjects will be required to stay on the GCRC for a 17-day period. This extended hospital stay is fundamental in order to limit illicit drug use during the study period. The services we will need from the GCRC include clinical admitting procedures; obtaining urine toxicology screen before each test probe dose; placement of IV catheters; monitoring vital signs at least one time per shift and at more frequent intervals from 12 to 24 hours post smoked-cocaine administration; monitoring study drug infusion; collecting, processing, and packaging study-mandated urine and blood samples; and administering several subjective questionnaires. Performing these functions will aid in obtaining accurate study data which is vital for the successful completion of this study.